In the field of electric arc welding, there are a number of criteria upon which an electrode is judged and used by the welding industry.
The first is that the electrode must produce a solid, non-porous weld bead which has the tensile strengths and Charpy Impact values required of the end use to which the weld will be put. Porosity is usually caused by the nitrogen of the air combining, in the heat of the arc, with the metals of the electrode as they transfer from the electrode to the weld pool, and then being released as the deposited weld metal cools. Nitrogen is normally prevented from coming in contact with the arc by including appropriate fluxing materials, such as the metal fluorides, in the core of the electrode and/or, and in the case of the present invention, by additionally surrounding the end of the electrode, the arc and the molten weld pool with a flowing stream of nitrogen free gases such as argon, helium, carbon dioxide, or mixtures of the same.
To obtain the required tensile strengths and Charpy Impact Values, the flux in the core of the electrode normally includes various known steel alloying ingredients such as chromium, nickel, manganese, boron, and silicon in appropriate amounts, such metals being added either as elements, ferro alloys, or alloys of themselves, or as oxides in combination with suitable reducing agents.
Another important criterion of welding electrodes is the ability to weld both downhand and out-of-position, e.g., vertical up or overhead. For downhand welding, the flux in the core of the electrode contains various known slag forming ingredients which float on the molten weld beads as it cools protecting it from the atmosphere and then solidify after the weld metal solidifies. These ingredients are so proportioned that the solidifying temperature of the mixture is below the solidifying temperature of the weld bead so as not to adversely affect the shape of the surface thereof. For out-of-position welding, these slag forming materials not only must coat and shape the weld bead as it solidifies, but must have substantially higher viscosity than the fluxing materials used for downhand welding so as to support the molten weld metal in position against the force of gravity.
This latter requirement is somewhat incompatible with another criteria of the acceptability of a welding electrode, namely the physical appearance of the surface of the weld bead after the weld bead has solidified and the slag has been removed. Thus, it is desirable to have a weld bead which has a generally flat surface rather than a humped surface which is wasteful of weld metal and must sometimes be ground off at substantial expense. Also, the bead desirably should have a shiny surface.
A further criteria is easy removal of the hardened slag.
These last two criteria can only be met by proper formulation of the slag forming materials.
In the past, in order to optimize bead appearance for downhand welding, some degradation of bead shape has been accepted for out-of-position welding. The converse compromise is often made in formulation optimized for out-of-position welding in that out-of-position electrode formulations may sacrifice downhand weld bead appearance.
Formulations used by applicants' assignee for flux cored electrodes to be used with external gas shielding have included substantial amounts of the metal oxides, such as titanium dioxide, silicon dioxide and sodium oxide; lesser amounts of a fluoride such as calcium fluoride, and known steel alloying elements such as boron, carbon, silicon, titanium, vanadium, chromium, manganese, nickel, zirconium and the like.
Specifically, one such electrode included the following range of ingredients:
______________________________________ Maximum Minimum % Elec. % Elec. % Fill Total % Fill Total ______________________________________ Na.sub.2 O 3.40 0.51 3.02 .45 Al.sub.2 O.sub.3 -- -- -- -- SiO.sub.2 5.35 0.80 3.90 0.58 TiO.sub.2 57.54 8.63 52.94 7.94 ZrO.sub.2 1.03 0.15 -- -- CaF.sub.2 1.09 0.16 0.87 0.13 B 0.026 0.004 0.021 0.0032 Si 5.89 0.884 5.32 0.798 Mn 17.79 2.67 16.93 2.54 Fe 12.0 1.80 ______________________________________
Iron in powder form is a filler which melts and forms part of the deposited weld metal.
It may be noted that this formulation includes no metallic magnesium.
Such electrode while producing excellent out-of-position welds tended to produce a humped bead in the downhand position which had a dull surface.